Process of oxyalkylation employing solid, heterogeneous oxyalkylation catalysts

ABSTRACT

Oligomeric polyoxyalkylene polyethers are prepared by oxyalkylating a low molecular weight hydroxyl-functional starter molecule with one or more alkylene oxides in the presence of a solid, heterogeneous magnesium oxide catalyst. The catalyst is readily and rapidly removed by simple filtration to yield a polyether with minimal metal ion content suitable for use directly or as a starter molecule for further double metal cyanide complex catalyzed oxyalkylation.

This is a division, of application Ser. No. 08/554,010, filed Nov. 6,1995.

TECHNICAL FIELD

The present invention pertains to oxyalkylation of suitably hydricinitiator molecules to produce oligomeric polyoxyalkylene polyethers.More particularly, the present invention pertains to oxyalkylation inthe presence of solid, heterogeneous catalysts which can be readilyremoved from the polyoxyalkylene polyether product without resorting toneutralization or adsorbent treatment.

BACKGROUND ART

Polyoxyalkylene polyols have a myriad of industrial uses. Monofunctionalpolyoxyalkylene polyethers have applications as surface active agents,reactive plasticizers, and the like. Di- and polyfunctionalpolyoxyalkylene polyethers (polyether polyols) may be used to preparepolyesters by reaction with dicarboxylic acids or their derivatives, andparticularly, a wide variety of polyurethane polymers by reaction withan organic di- or polyisocyanate.

In the past, oxyalkylation of a suitably hydric initiator such as amonofunctional alkanol or polyfunctional diol, triol, or the like, wasperformed with homogenous, highly basic catalysts such as sodium orpotassium hydroxide or the corresponding alkoxides. Under the reactionconditions employed, oxyalkylation with propylene oxide is accompaniedby a competing rearrangement of propylene oxide to allyl alcohol, amonohydric initiator which competes with the desired initiator foroxypropylation with propylene oxide. This rearrangement is discussed inBLOCK AND GRAFT POLYMERIZATION, Ceresa, Ed., John Wiley & Sons, NewYork, at pages 17-21, and to date, the mechanism is still subject todebate. Whatever the mechanism, the result is continued generation ofmonofunctional, allylic unsaturation-containing polyoxypropylene monols.

During preparation of monofunctional polyoxypropylene polyethers, theeffect of the continued generation and subsequent oxypropylation ofallyl alcohol does not alter the average functionality of the product,but the average molecular weight is lowered and the molecular weightdistribution (polydispersity) altered in generally undesirable ways.Moreover, the reactive allylic unsaturation is prone to oxidation aswell as a variety of addition reactions. When the initiator is a longchain alkanol, desired to impart hydrophobic character, the high molpercentage of relatively short allyl alcohol moieties can drasticallyalter surface active properties in high molecular weight non-ionicsurfactants.

In the case where higher hydric initiators with functionalities of e.g.2 to 8 or higher are used, as is generally the case for polyoxyalkylenepolyethers for polyurethanes, the propylene oxide/allyl alcoholrearrangement is far more deleterious. For example, oxypropylation ofpropylene glycol or dipropylene glycol to produce polyoxypropylene diolsis generally limited to products having a molecular weight of c.a. 4000Da, or a 2000 Da equivalent weight product. Even at this modestequivalent weight, such a polyoxypropylene diol will contain up to 50mol percent monol. The average functionality of the polyether product islowered from a theoretical, calculated functionality of 2.0, to ameasured functionality of from 1.5 to 1.7.

In addition to the above-mentioned drawbacks associated with the use ofbasic oxyalkylation catalysts, in general, the oxyalkylation catalystmust be removed and/or neutralized. In general, removal is facilitatedby treatment of the polyether product with an adsorbent, for example,magnesium silicate, followed by filtration. Use of such adsorbentsincreases costs, and the filtration adds undesirably to the processtime, thus lowering throughput or requiring additional capitalization toprovide separate vessels to hold the product being filtered. Themagnesium silicate filter cake is sometimes pyrophoric, and in any case,must be disposed of by environmentally acceptable methods.

Many efforts have been made to reduce the unsaturation in polyetherpolyols by minimizing allyl alcohol formation. To this end, it has beenproposed in U.S. Pat. No. 3,393,243 to employ cesium hydroxide orrubidium hydroxide as catalysts. However, these catalysts, althoughsomewhat effective in reducing unsaturation, are far more expensive thantheir lighter alkali metal analogs, and still require adsorbenttreatment for removal. Furthermore, due to the higher atomic weight ofthe alkali metal involved, a greater weight percentage must be used toprovide the same mol percent of catalyst. In U.S. Pat. No. 4,282,387,the use of calcium, strontium, or barium carboxylates such as calciumnaphthenate is disclosed, without subsequent residual catalyst removal.However, the effect on unsaturation is undocumented, and the high levelsof residual catalyst are unsuited for many applications, particularlythose related to coatings or containers for food products.

In U.S. Pat. Nos. 5,010,187 and 5,114,619 are disclosed the use ofhigher alkaline earth metal oxides and hydroxides, for example bariumand strontium oxides and hydroxides, as oxyalkylation catalysts.Unsaturation is lowered somewhat, however, catalyst residues must stillbe removed via neutralization and/or adsorption. Use of catalystscontaining strontium requires particular emphasis on catalyst removaldue to the documented toxicity of strontium.

In the decades of the 1960's and 70's, double metal cyanide complex (DMCcomplex) catalysts were introduced for oxypropylation. These catalysts,such as a non-stoichiometric zinc hexacyanocobaltat-glyme complex, werefound to be highly efficient catalysts for oxypropylation under certainconditions. Polyether polyols of much higher molecular weight thanpreviously available could be produced. For polyoxypropylene diolshaving equivalent weights of c.a. 2000 Da, the unsaturation was in therange of 0.015 to 0.020 meq/g as compared to 0.07 to 0.09 meq/g forotherwise similar base-catalyzed polyols.

However, the oxyalkylation of hydric initiators with DMC complexcatalysts has been found to be inefficient when the molecular weight ofthe initiator molecule is below about 400 Da. For example, propyleneglycol, glycerine, and trimethylolpropane, all common initiators inbase-catalyzed oxyalkylation, are generally unacceptable when DMCcomplex catalysts are utilized. In their stead, oligomericpolyoxyalkylene polyethers based on such initiators must be used. Sucholigomeric products, for example a 450 Da molecular weight oxypropylatedglycerine, must be first prepared by conventional methods such as basecatalysis. However, the basic oxyalkylation catalysts used to preparethe oligomeric starter molecules must be removed prior to further DMCcomplex catalyzed oxyalkylation, as they are known to poison orotherwise inhibit DMC complex catalysts. The removal of catalystresidues, as indicated before, adds unwanted time and expense to theoverall process.

It would be desirable to provide a process for oxyalkylation,particularly oxypropylation, whereby oligomeric polyoxyalkylenepolyethers may be produced with minimal treatment to remove catalystresidues. Such oligomeric polyether products are useful in and ofthemselves, and are particularly useful as initiator molecules forfurther oxyalkylation in the presence of DMC complex oxyalkylationcatalysts.

SUMMARY OF THE INVENTION

It has now been surprisingly discovered that magnesium oxide is asuitable oxyalkylation catalyst for preparation of oligomericoxyalkylation products, despite the catalyst being heterogeneous. It hasfurther been surprisingly discovered that magnesium oxide catalysts arenot solubilized in the oxyalkylation product, and may be removed byrapid, simple filtration, to provide a polyether product with minimalmetal ion content. Most surprisingly, these catalysts are far moreeffective than basic metallic oxides such as aluminum oxide and calciumoxide, despite the latter's higher alkalinity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The oxyalkylation catalysts of the subject invention comprise magnesiumoxide, which is commercially available in a variety of particle sizes.Preferred particles sizes range from 100 mesh to 1000 mesh, with sizesin the range of 200-500 mesh preferred. Smaller particle sizes providelarger surface area and enhanced dispersibility, the latter thereforerequiring less intensive stirring. However, filtration is most rapid andefficient with larger particle sizes. The heterogeneous nature of thecatalyst requires that the reaction mixture be agitated, or recirculatedto the oxyalkylation reactor, or that the reactants flow through a fixedor fluidized catalyst bed.

The preferred alkylene oxide used in oxyalkylation is propylene oxide,although other alkylene oxides such as 1,2- and 2,3-butylene oxide,ethylene oxide, and higher C₅₋₂₀ alkylene oxides may be used as well,either singly or in admixture. Mixtures of propylene oxide and ethyleneoxide are also preferred. When use of more than one alkylene oxide iscontemplated, the alkylene oxides may be added at the same time, orsequentially in any order, to form block, random, block-random, andother oxyalkylation products. The amount of alkylene oxide mayadvantageously range from 0.5 mol to about 10 mol on a hydroxylequivalent basis, i.e., from 0.5 to 10 mol per mol of hydroxylfunctionality in the initiator. Preferably, the amount of alkylene oxideis from 0.5 mol to 5 mol, more preferably 0.8 to 2.5 mol on theaforementioned basis.

The oxyalkylation temperature and pressure are conventional, i.e.,similar to that used in conventional base catalysis using potassiumhydroxide. Temperatures in the range of about 50° C. to 220° C. aresuitable, preferably 70° C. to 160° C., and most preferably in the rangeof 90° C. to 120° C. Pressures may range from below atmospheric to 100psi, preferably from 20 to 90 psi.

The time of oxyalkylation is dependent on the amount of alkylene oxidedesired to be reacted, and inversely related to the catalyst charge andoxyalkylation temperature. Following completion of the addition of thedesired amount of alkylene oxide, the reactor pressure may be monitoredto provide an indication of the progress of oxyalkylation. The reactormay be maintained at a suitable oxyalkylation temperature untilsubstantially all alkylene oxide has reacted, or may be vented, allowingunreacted alkylene oxide to escape the reaction vessel. Alkylene oxideaddition times advantageously range from 0.5 h to 10 h, preferably 1-9hours, with "cook-out" times of 0.5 h to 16 h, preferably 0.5 h to 5 h.Addition and cook-out time may be decreased by using a larger amount ofcatalyst, possible due to the inexpensiveness of the catalyst and itsrecyclability.

Suitable initiator molecules have functionalities of from 1 to 8 andhigher, preferably 2 to 8. The "hydric" functionality is preferablyhydroxyl functionality. The initiator molecules are preferably"monomeric" in the sense that they have not been previouslyoxyalkylated. However, low molecular weight oligomeric oxyalkylationproducts of such monomeric initiator molecules may be used as well.Non-limiting examples of suitable monomeric initiator molecules includemono- alkanols such as methanol, ethanol, 1-propanol, 2-propanol,1-butanol, 2-butanol, 2-ethylhexanol, and the like; aliphatic, aromaticand arylaliphatic diols such as ethylene glycol, propylene glycol, 1,2-and 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol,2,2,4-trimethylpentane-1,5-diol, neopentyl glycol, 1,2-, 1,3-, and1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, hydroquinone,resorcinol, 4,4'-dihydroxybiphenyl, the bisphenols, e.g., bisphenol A,bisphenol F, and bisphenol S; triols such as trimethylolpropane andglycerine; tetrols such as pentaerythritol; pentols such asα-methylglucoside; hexols such as the simple saccharides, e.g., glucose,fructose, mannose, or sorbitol, and modified saccharides such ashydroxyethylglucoside, and hydroxypropylglucoside; octols such assucrose; and the like.

Low molecular weight oligomers include diethylene glycol, triethyleneglycol, dipropylene glycol, tripropylene glycol, dipentaerythritol,tripentaerythritol, and the like. Higher molecular weight oligomersinclude the alkylene oxide adducts of the foregoing initiators or aminesor diamines such as N,N,N',N'-tetrakis 2-hydroxypropyl!ethylene diamineand the like, with molecular weights of from about 100 Da to about 1500Da and higher, preferably 100 Da to 1000 Da, especially polyoxyethyleneglycols, polyoxypropylene glycols, glycerine-initiated polyoxyethyleneand polyoxypropylene polyether polyols, and the like. Preferredinitiators are ethylene glycol, diethylene glycol, propylene glycol,dipropylene glycol, glycerine, trimethylolpropane and polyoxyethylatedand/or polyoxypropylated polyether oligomers of these initiators havingmolecular weights in the range of 100 Da to 1500 Da and/or hydroxylnumbers in the range of 3700 to about 100, preferably 1829 to 100. Themolecular weights and equivalent weights expressed in Da (Daltons)herein are number average molecular weights.

The reaction advantageously is conducted in a stainless steel orglass-lined pressure vessel equipped with the necessary feed ports and amechanical stirrer, or a similar vessel in which reactor contents arecontinuously or discontinuously recirculated via a recirculation pump.Due to the heterogeneous nature of the catalyst, a continuous reactoremploying a fixed bed or fluidized bed of solid catalyst may beemployed, with initiator introduced at an inlet end along with a firstportion of alkylene oxide, with increments of alkylene oxide added atpoints downstream from the inlet, and product take-off at an outlet end.

The solid oxyalkylation catalysts of the subject invention may beintroduced directly into the oxyalkylation reactor, if desired. Toassure complete freedom from moisture, the catalysts may be heated todrive off any moisture which they may have absorbed or adsorbed duringstorage. The catalysts are preferably heated in vacuo for this purpose.Alternatively, the catalyst/starter mixture may be heated in vacuo inthe reactor to remove traces of water. In the case where monomeric diolssuch as ethylene glycol or propylene glycol are used as startermolecules, removal of water may not be required.

Initiator/catalyst master batches may be advantageously used, preparedas described above, and stored in a sealed container until use. Tofacilitate thorough mixing of catalyst and initiator (starter), a lowboiling solvent, preferably one having water solubility or miscibility,may be added to the mixture of catalyst and initiator, following whichthe solvent is removed by vacuum stripping or distillation. Suitablesolvents are, for example, acetone, methylethylketone,N-methylpyrollidone, and tetrahydrofuran.

Following oxyalkylation, the catalyst is filtered from the polyetherproduct. As the catalyst particles are preferably of rather large size,e.g. 200 to 500 mesh, filtration is rapid, and may be accomplished bystandard industrial filters such as bag filters, cartridge filters,plate and frame filters, and the like. Centrifugal filters may also beused, as well as combinations of coarse prefilters designed to retainthe bulk of the catalyst and a filter of finer pore size to retain anyfine particulates.

The catalyst may be recycled, if desired, to be used in a furtheroxyalkylation. When recycled, the catalyst may be washed with one of theaforementioned solvents to remove traces of polyether product whichmight otherwise remain in the filter cake, or may be a dded directly tothe oxyalkylation reactor. In the latter case, it is preferable to heatand vacuum strip to remove any traces of moisture which may haveaccumulated in the catalyst during filtration and/or handling.

The amount of catalyst will affect oxyalkylation time. Amounts ofcatalyst ranging upwards from about 1 weight percent based on productweight are suitable, with the maximum amount generally limited only bypractical concerns. In continuous flow reactors, for example, the amountof catalyst may actually be larger than the amount of reactants andproducts contained in the reactor at any given time. In the case ofbatch-type reactions, whether in stirred pressure vessels orrecirculating reactors, the amount of catalyst, again based on finalproduct weight, may be from 1 weight percent to 50 weight percent,advantageously 3 weight percent to 30 weight percent, and preferably 5weight percent to 25 weight percent. Amounts of from about 8 weightpercent to 15 weight percent are particularly suitable.

Conventional reactor preparation is utilized. Following introduction ofthe initiator and solid catalyst charges, the reactor may be heated andstripped, preferably at elevated temperature, to remove traces of water,and purged several times with dry nitrogen. The alkylene oxide is thenadded with continuous agitation in the case of a stirred reactor, orcontinuous flow in the case of a recirculating or continuous reactor.Following conclusion of the oxyalkylation, the crude product is filteredto remove traces of solid catalyst. The filtered polyol contains verylittle residual metal ions, in general less than 5 ppm, and inparticular, less than 1 ppm. The product may be used as is, or may befurther oxyalkylated to higher molecular weight products using DMCcomplex catalysts, without post treatment using acid neutralization, ionexchange treatment, or adsorbents to remove residual catalyst. Polyolsprepared in this manner have measured unsaturation of less than 0.020meq/g, preferably less than about 0.015 meq/g, more preferably less thanabout 0.010 meq/g, and in particular, 0.007 meq/g or less.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLE 1

To 200 glycerine (hydroxyl No. 1829) in a rotoevaporator flask was added100 g MgO (Aldrich, 325 mesh, calcined, 99+ % purity) and 200 gtetrahydrofuran (THF). The rotoevaporator contents were mixed well andTHF removed in vacuo. An additional 200 g glycerine was then added toform an initiator/catalyst mixture containing 20 weight percentmagnesium oxide. A stainless steel stirred autoclave was charged with200 g of the initiator/catalyst mixture, pressurized with dry nitrogento 30 psig and purged to full vacuum (-14 psig), this procedure beingrepeated a total of three times. Full vacuum was applied to the reactoras it was heated to 110° C. with the stirrer at 733 rpm. Propylene oxidewas added at a pressure of from 70-90 psi over a period of 9 hours,following which a cook-out of 16 hours at 130°-140° C. was used to reactthe majority of propylene oxide. The reactor was vented, and thecontents were cooled and filtered. The resulting polyol product had ahydroxyl number of 643, an unsaturation of 0.003 meq/g polyol, andcontained less than 1 ppm magnesium.

EXAMPLES 2 to 6

In a manner similar to Example 1, polyols were produced from a varietyof initiators with magnesium oxide as catalyst. The reaction conditionsand product physiochemical properties are presented in Table 1. In Table1, the build ratio is the calculated ratio of product molecular weightto initiator molecular weight. M_(n) and M_(w) are the number averagemolecular weight and weight average molecular weight, respectively, ofthe product, while M_(w) /M_(n) is the product polydispersity.Unsaturation is measured by titration in accordance with ASTM testmethod D-2849-69. Percent catalyst is the weight percent relative to theobtained product weight.

                                      TABLE 1                                     __________________________________________________________________________    EXAMPLE   1    2    3    4    5    6                                          __________________________________________________________________________    Type      Slurry                                                                             Slurry                                                                             Slurry                                                                             Slurry                                                                             Slurry                                                                             Slurry                                     Reactor Temp (°C.)                                                               110  160  120  110  100  160                                        STARTER                                                                       OH#       1829.sup.1                                                                         264.sup.4                                                                          108.sup.3                                                                          238.sup.2                                                                          108.sup.3                                                                          1829.sup.1                                 Charge (g)                                                                              160  214  216  160  200  160                                        Catalyst  MgO 325                                                                            MgO 325                                                                            MgO 325                                                                            MgO 325                                                                            MgO 325                                                                            MgO 325                                    Catalyst (g)                                                                            40   54   54   80   100  40                                         Build Ratio                                                                             3.00 1.87 1.89 3.76 1.50 4.00                                       PO Feed Time (hour)                                                                     9.00 2.00 4.00 9.00 4.30 8.00                                       Cook-Out (hour)                                                                         16.00                                                                              1.00 5.00 7.00 0.50 3.50                                       OH# Polyol                                                                              643  166  82   73.7      450                                        Unsat. Polyol  0.077                                                                              0.063                                                                              0.083                                                M.sub.n        740  1320 6574                                                 M.sub.w        850  1640 8733                                                 M.sub.w /M.sub.n                                                                             1.13 1.24 1.328                                                % Catalyst                                                                              6.7  10.7 10.6 8.8  22.0 5.1                                        __________________________________________________________________________     .sup.1 Glycerine;                                                             .sup.2 LHT240, an oxypropylated glycerine available from ARCO Chemical        Co.;                                                                          .sup.3 PPG1025, polyoxypropylene diol with nominal 1000 Da molecular          weight;                                                                       .sup.4 PPG 425, polyoxypropylene diol, c.a. 425 Da molecular wt.         

COMPARATIVE EXAMPLE A

Calcium hydroxide was calcined at 400° C. for 6 hours to prepare acalcium oxide catalyst. Catalyst in the amount of 150 g was slurried in300 g PPG-1025 and 360 g of this mixture oxypropylated with propyleneoxide as described in Examples 1-6, the reactor temperature being 110°C. The reaction was allowed to proceed for in excess of 4 days.Following stripping at maximum vacuum, crude product weight (includingcatalyst) was only 563 g, representing approximately 443 g polyol afterfiltering (Whatman #1 filter paper) to remove catalyst. Due to the smallamount of weight increase and long reaction time, the polyol product wasnot analyzed.

COMPARATIVE EXAMPLE B

Basic aluminum oxide (Aldrich), 150 g, was employed as a catalyst with700 g PPG-1025 in a plug flow reactor. Reaction temperature was 140° C.,flow rate at 50 ml/min, and propylene oxide feed rate at 3.0 g/min. Atthis rate of feed, pressure would rise to c.a. 60-70 psig at which timethe propylene oxide feed was interrupted and allowed to "cook out" to alower pressure following which feed was restarted. The reactor wasstripped, cooled, and product collected. The product, followingrefining, has a hydroxyl number of c.a. 108, approximately the same asthe initiator polyol. The crude product gelled over time but could beclarified upon heating to 100° C. for 2 hours, only to gel once moreupon cooling. Refined product also gelled.

EXAMPLE 8

Potassium hexacyanocobaltate (8.0 g) was added to deionized water (150mL) in a beaker, and the mixture blended with a homogenizer until thesolids dissolved. In a second beaker, zinc chloride (20 g) was dissolvedin deionized water (30 mL). The aqueous zinc chloride solution wascombined with the solution of the cobalt salt using a homogenizer tointimately mix the solutions. Immediately after combining the solutions,a mixture of tert-butylalcohol (100 mL) and deionized water (100 mL) wasadded slowly to the suspension of zinc hexacyanocobaltate, and themixture homogenized for 10 min. The solids were isolated bycentrifugation, and were then homogenized for 10 minutes with 250 mL ofa 70/30 (v:v) mixture of t-butylalcohol and deionized water. The solidsare again isolated by centrifugation, and finally homogenized for 10minutes with 250 mL of t-butylalcohol. The catalyst was isolated bycentrifugation, and dried in a vacuum oven at 50° C. and 30 in. (Hg) toconstant weight.

A high molecular weight, low unsaturation polyether polyol is preparedusing double metal cyanide complex catalyzed oxyalkylation of theoligomeric polyether prepared in accordance with Example 1. A two-gallonstirred reactor is charged with the c.a. 650 Da polyoxypropylene triolstarter prepared in accordance with Example 1 and the zinchexacyanocobaltate complex catalyst at a level corresponding to 250 ppmin the finished polyol. The mixture is stirred and heated to 105° C.,and is stripped under vacuum to remove traces of water from the starter.A minor amount of propylene oxide is fed to the reactor, initially undera vacuum of 30 in. (Hg), and the reactor pressure is monitoredcarefully. Additional propylene oxide is not added until an acceleratedpressure drop occurs in the reactor; the pressure drop is evidence thatthe catalyst has become activated. When catalyst activation is verified,sufficient propylene oxide to result in a 6000 Da polyoxypropylene triolproduct is added gradually over about 2 h while maintaining a reactorpressure less than 40 psi. After propylene oxide addition is complete,the mixture is held at 105° C. until a constant pressure is observed.Residual unreacted monomer is then stripped under vacuum from the polyolproduct. The hot polyol product is filtered at 100°0 C. through a filtercartridge (0.45 to 1.2 microns) attached to the bottom of the reactor toremove the catalyst. The product polyol has a hydroxyl number of c.a. 28and a measured unsaturation of less than about 0.005 meq/g unsaturationper gram of polyol.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed is:
 1. A process for the preparation of apolyoxyalkylene polyether having low unsaturation, comprising:(a)preparing an oligomeric polyoxyalkylene polyether by;(1) contacting ahydroxyl-functional starter having a first molecular weight with acatalytically effective amount of a solid, essentially insolubleoxyalkylation catalyst consisting essentially of magnesium oxide havinga particle size of from 100 to 1000 mesh, wherein said amount ofmagnesium oxide is about 1 weight percent or more based on the weight ofsaid oligomeric polyoxyalkylene polyether; (2) oxyalkylating saidstarter with one or more alkylene oxides to form an oligomericpolyoxyalkylene polyether of second molecular weight, said secondmolecular weight being higher than said first molecular weight; and (3)separating said solid oxyalkylation catalyst from said oligomericpolyoxyalkylene polyether; (b) adding to said oligomeric polyoxyalkylenepolyether a catalytically effective amount of a double metal cyanidecomplex catalyst; (c) oxyalkylating said oligomeric polyoxyalkylenepolyether with one or more alkylene oxides; and (d) recovering apolyoxyalkylene polyether having an unsaturation lower than 0.020 meq/gand a molecular weight higher than the molecular weight of saidoligomeric polyoxyalkylene polyether.
 2. The process of claim 1 whereinsaid oxyalkylating step (c) is performed with an alkylene oxide selectedfrom the group consisting of propylene oxide, and a mixture of propyleneoxide and ethylene oxide.
 3. The process of claim 1 wherein said doublemetal cyanide complex catalyst is a non-stoichiometric substantiallyamorphous catalyst having zinc hexacyanocobaltate and t-butyl alcohol.